Colored anodized aluminum is used in a wide variety of applications ranging from novelties and giftware through automotive trim to building and architectural components. Many organic dyes can be used to color anodized aluminum. The principal dyes are mordants of premetallized azo and anthraquinone type dyes having sulfonic, carboxylic, hydroxyl, amino, nitro, nitroso, and other substituents such as premetallized chromium chelate or a complex of a black mordant azo dye having the general formula 2-amino-4 nitrophenol.fwdarw.4,6,7-Trihydroxy-2-napthalene sulfonic acid.
The process of dyeing the aluminum oxide film of anodized aluminum comprises immersion of the anodized aluminum in an aqueous dye bath having about 1 wt % organic dye at 150.degree. F. for about ten minutes. The colored aluminum is then sealed to prevent dyeing and staining. In theory, with only makeup of the dye used for coloring, the dye bath should last indefinitely, In practice, however, the dye bath becomes contaminated with aluminum and other metal cations which increase the time required for dyeing and cause precipitates to be formed in the dye bath that result in off-quality product. The loss in production capacity, the high cost of the dyes and cost for disposal of the spent dyes dictate the need for a method to purify and restore the dye bath that provides consistent dyeing and maximum use of the dyes. Heretofore there has been no method for purification and restoration of the dye baths without some oxidation of the organic dye.
Electrodialysis is a well known art (see U.S. Pat. Nos. 4,325,792; 4,439,293; 3,896,013 and 3,964,985 the disclosures of which are hereby incorporated by reference). Electrodialysis is the transport of ions through ion permeable membranes as a result of an electrical driving force. The process is commonly carried out in an electrolytic cell having an anolyte compartment containing an anode and anolyte and a catholyte compartment containing a cathode and catholyte with the anolyte and catholyte compartments being separated by ion permeable membranes. An electric current is passed between the anode and cathode of the cell through the aqueous anolyte and catholyte solutions. The electric current causes, for example, the cations present in the anolyte to migrate through a cation permeable membrane into the catholyte. When the electrical potential, i.e., cell voltage, is sufficient to electrolyze water, oxygen is formed at the anode. My U.S. Pat. Nos. 4,325,792 and 4,439,293 teach electrotransport of multivalent cations through cation permeable membranes into aqueous solutions containing agents that insolubilize the multivalent cations.